Titanium and titanium alloys have become important structural metals due to an unusual combination of properties. These alloys have strength comparable to many stainless steels at much lighter weight. Additionally, they display excellent corrosion resistance, superior to that of aluminum and sometimes greater than that of stainless steel. Further, titanium is one of the most abundant metals in the earth's crust, and as production methods become more economical, will be employed in ever growing applications. Various alloys of nickel and titanium are part of the alloy class known as Shape Memory Alloys (SMAs). This term is applied to that group of metallic materials that demonstrate the ability to return to a defined shape or size with thermal processing. In a most general sense, these materials can be plastically deformed at some relatively low temperature and return to their pre-deformation shape upon some exposure to higher temperatures.
SMAs have been observed for more than 70 years in a wide range of alloys, such as AuCd, CuZn, FePt, and FeMnSi. Although a wide variety of alloys have been observed to demonstrate the shape memory effect, only those that either generate significant force or are able to recover substantial amounts of strain are of commercial interest. Currently, this has generally been the nickel-titanium (NiTi) alloys, including Nitinol (an acronym for NIckel TItanium Naval Ordinance Laboratory) alloys, and such copper based alloys as CuZnAl and CuAlNi. Nickel-titanium, for example, is commercially available in such diverse forms as wire, ribbon, tubing including microtubing, sheet, and can be formed into rods, bars, solid wire, stranded wire, braided wire, sputtering targets, and thin films for use in a wide variety of industries
SMAs undergo a phase transformation in their crystalline structure when cooled through a transition temperature from the relatively stronger, high temperature or “Austenite (or austenitic)” form to the relatively weaker, low temperature or “Martensite (or martensitic)” form. Such crystalline transformations are responsible for the hallmark characteristics of these materials; their thermal, or shape, memory; and their mechanical memory. When a SMA is in its low temperature, or martensitic, form, it can be easily deformed into a new shape. If the deformed material is heated through a transformation temperature, the material reverts to the higher temperature, or austenitic, form. The material regains its original shape, sometimes reverting in shape with great force. Very slight differences in the alloy composition of the SMA can considerably affect the transition temperature for an alloy, as can heat treatment of the alloy. The shape memory effect takes place over a range of just a few degrees and the transformation effect can be controlled to take place within a degree or two of desired temperature.
“Mechanical memory” is demonstrated if the SMA is deformed at a temperature which is slightly above the transformation temperature. This effect is caused by stress induced martensitic formation. The martensitic material will revert immediately to the undeformed austenitic form as soon as the stress is removed. This makes the material highly elastic and rubber-like, and able to recover up to approximately 8% recoverable strain.
The “thermal memory” of these alloys, that is, their tendency to return to a predetermined shape after thermal processing, is not qualitatively different from their “mechanical memory,” that is, their tendency to elastically deform, and then to return to their original configuration, when held at a constant temperature. This mechanical memory is also called “superelasticity” or “pseudoelasticity.” This property of superelasticity observed in SMAs has led to widespread commercial use in such diverse fields as cellular telephone antennas, eyeglass frames, women's brassieres, fishing lures, and medical devices. The area of medical devices has been of particular interest, as nickel-titanium alloys have shown a high degree of biocompatibility, corrosion resistance under physiologic conditions, and excellent cytocompatibility. Additionally, nickel-titanium has a lower magnetic susceptibility than stainless steel, making it compatible with MRI (Magnetic Resonance Imaging) systems. Superelasticity allows the passage of a complex instrument through a narrow cannula, and then to have the instrument elastically regain its desired conformation upon exiting the cannula. Applications include, by way of example and not limitation, right-angle needles, suture passers, retractors, graspers, baskets, and various retrieval devices. Since the nickel in these alloys is chemically bound to the titanium in a strong intermetallic bond, risks of human tissue reaction have been shown to be low.
A major limitation in the use of nickel-titanium alloys has been the difficulty of joining this material, both to itself, and to other materials. Because of its high cost, it is often desirable to limit the use of nickel-titanium to the actual moving parts of a device, while fabricating supporting members of such materials as stainless steel. However, welding of nickel-titanium to stainless steel has proved particularly troublesome, as disclosed by Ge Wang, in a review “Welding of Nitinol to Stainless Steel.”
Fusion welding has been fraught with difficulties, particularly, problems surrounding issues of solidification, or “hot,” cracking, and cracking due to intermetallic formation, or so-called “cold” cracking. Hot cracking is due to inherent characteristics of alloys. Unlike pure metals, alloys solidify through a range of temperatures, rather than at a single melting point. This temperature range, called the freezing zone or mushy zone, is a temperature zone in which the high melting point constituents of the alloy solidify first and form a continuous interlocking solid network. During the cooling of welds, the alloy, both liquid and solid, continuously shrinks in volume, so that a tensile force is constantly applied across a solid network that is interlaced by a thin liquid film. This tensile force causes cracks to form at the liquid metal filled grain boundaries, and these cracks then propagate through the weld zone. The larger the mushy zone, the more severe the solidification cracking problem. Low melting point impurities such as phosphorous (P) or sulfur (S) can contribute to hot cracking. For example, S in as low a concentration of 9 ppm in a nickel alloy can be sufficient to cause hot cracking.
“Cold” cracking is a particular problem when attempting to weld nickel-titanium to other materials, and is responsible for common observations in the art that welding is generally not an acceptable method of joining nickel-titanium to other materials, e.g., stainless steel, because brittle intermetallics are formed in the weld zone. Ti and Fe form the brittle intermetallic compounds TiFe and TiFe2, both of which can cause cold cracking at welded joints. The compressive strength of the intermetallics compounds TiFe and TiFe2 is virtually zero due to their extreme brittleness. Techniques such as direct fusion welding cause intermetallic formation at the bond line, and consequential failure of the weld. Even solid state bonding techniques which do not require melting at the weld interface, while they initially form a stronger weld, are susceptible to solid state diffusion of intermetallics into the weld line, and consequential weakening.
In addition, the reactivity of titanium makes it important that any welding be done in a clean, inert atmosphere or in a vacuum, to reduce the tendency to form damaging oxides or nitrides. Nickel-titanium materials naturally form surface oxides in air during processing into finished form. The principal surface oxide formed is TiO2.
Compared with attempts to weld nickel-titanium to ferrous metals, more success has been experienced with joining nickel-titanium to itself with such techniques as laser welding, plasma welding, resistance welding, and e-beam welding. Subtle variations in the composition of the nickel-titanium alloys greatly affect the inherent stability of homologous nickel-titanium welds. For example, nickel rich nickel-titanium alloys, such as those that are comprised of approximately 50.5% nickel, are readily weldable to itself by the above techniques. On the other hand, titanium rich nickel-titanium alloys, such as those composed of approximately 51.5% titanium, are susceptible to solidification cracking. The instant inventor showed improvement in homologous nickel-titanium welds with the addition of nickel to the weld pool in his review, “Resistance Welding Ti-Rich Nitinol Wire.” Grain boundaries have been shown to be still wet with liquid during the last stages of solidification and are easily separated by thermal shrinking stresses. As a result, cracks form at the weld metal centerline.
However, the difficulty of joining nickel-titanium to other materials, such as stainless steel, has remained exceedingly limiting to the art. Many techniques have been employed with limited success. Non-fusion joining methods are most commonly used to join nickel-titanium; including soldering, epoxies and other adhesives; and various types of mechanical joining such as crimping. These techniques are not without their problems. Soldering, for example, must often be accomplished with special flux to remove and inhibit the formation of surface oxides during soldering. Epoxies and adhesives are not suitable for all manufacturing techniques and types of uses to which these nickel-titanium products are directed. Mechanical fastening may cause overdeformation and cracking of the nickel-titanium. Interference fit or the interlocking of components has been successful, but requires manufacturing to close dimensional tolerances.
Various methods have been used to attempt to improve results in welding of titanium alloys to ferrous metals. One such example is seen in U.S. Pat. No. 4,674,675 to Mietrach. The '675 method relies upon providing at least two intermediate metallic layers for placement between the titanium containing portion and the ferrous portion. The layer adjoining the titanium containing portion is vanadium and the layer adjacent to the ferrous metal is one of the group consisting of chromium, nickel, and iron. The resulting multilayer composition is then diffusion welded together. This approach suffers from the inherent complexity of a multilayer approach, which is disclosed in some embodiments to employ even more layers, consisting of tungsten and platinum, added to the vanadium and chromium/nickel/or iron layer. Additionally, the nature of diffusion welding makes the process quite slow and cumbersome, requiring approximately 90 minutes at a pressure of 10 Newtons per square millimeter to achieve a satisfactory diffusion weld.
Additionally, U.S. Pat. No. 3,038,988 to Kessler discloses the use of a vanadium interlayer between titanium and a ferrous metal, wherein the electrode pressure, the strength of the welding current, and the welding time are regulated such that an unmelted, or solid, core of the vanadium interlayer is preserved. This prevents intermixing of the ferrous and titanium elements and the consequent prevention of intermetallic formation; however, welding conditions must be strictly controlled in order to prevent the liquefaction of the vanadium interlayer, making this technique less suitable for production use. Following this practice of using a vanadium interlayer, U.S. Pat. No. 4,708,282 to Johnsen et al. teaches the use of a sintered material made of vanadium, titanium, and iron, allowing for complete melting of the weld metal without intermetallic formation. Such a method suffers from the additional steps involved in the complex manufacture of the tripartite weld metal.
Further, U.S. Pat. No. 6,410,165 teaches a method of nickel enriching the weld zone specifically directed to the welding of a high carbon, powder metallurgical, cobalt free tool steel that contains greater than 1 wt. % of carbon and total refractory metal additions greater than 15 wt. %.
Accordingly, the art has needed a means for improving the art of fusion welding titanium, and titanium based alloys, to ferrous metals. While some of the prior art devices attempted to improve the state of the art, none has achieved the unique and novel configurations and capabilities of the present invention. With these capabilities taken into consideration, the instant invention addresses many of the shortcomings of the prior art and offers significant benefits heretofore unavailable. Further, none of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.